Intraocular pressure in the eye is maintained by the formation and drainage of aqueous humor, a clear, colorless fluid that fills the anterior and posterior chambers of the eye. Aqueous humor normally flows from the anterior chamber of the eye out through an aqueous outflow channel at a rate of 2 to 3 microliters per minute.
Glaucoma is a progressive disease of the eye characterized by a gradual increase of intraocular pressure (IOP). This increase in pressure is most commonly caused by stenosis or blockage of the aqueous outflow channel, resulting in excessive buildup of aqueous fluid within the eye. Other causes include increase in venous pressure outside the eye which is reflected back through the aqueous drainage channels and increased production of aqueous humor. In a “normal” eye, IOP ranges from 8 to 21 mm mercury. In an eye with glaucoma, IOP can range between normal pressures up to as much as 50 mm mercury. This increase in IOP produces gradual and permanent loss of vision in the afflicted eye.
Existing corrective methods for the treatment of glaucoma include drugs, surgery, and implants. Pharmacological treatment is prohibitively expensive to a large majority of glaucoma patients. In addition, many people afflicted with the disease live in remote or undeveloped remote areas where the drugs are not readily accessible. The drugs used in the treatment, in particular steroids, often have undesirable side effects and many of the long-term effects resulting from prolonged use are not yet known. Additionally, patient compliance is an issue and, for example, patients often forget their medication.
Surgical procedures have been developed in an effort to treat victims of glaucoma. An iridectomy, removal of a portion of the iris, is often used in angle-closure glaucoma wherein there is an occlusion of the trabecular meshwork by iris contact. Removal of a piece of the iris then gives the aqueous humor free passage from the posterior to the anterior chambers in the eye. A trabeculotomy, opening the inner wall of Schlemm's canal, is often performed in cases of developmental or juvenile glaucoma so as to increase the outflow of the aqueous humor, thereby decreasing IOP. In adults, a trabeculectomy shunts fluid through a trap-door flap in the eye that performs a valve-like function for the first few weeks after surgery.
While often successful, these surgical techniques possess inherent risks associated with invasive surgery on an already afflicted or compromised eye. Furthermore, the tissue of the eye can scar over this small area and the eye reverts to the pre-operative condition, thereby necessitating the need for further treatment.
Ocular implants are often used in long-term glaucoma treatment. One early implant is described in the paper entitled “Use of Molteno Implants to Treat Secondary Glaucoma” by A. C. B. Molteno and published by Grune & Stratton, Ltd, 1986, pp 211-238. The implant was a small circular plate with a rigid translimbal drainage tube attached thereto. The plate was 8.5 mm in diameter and formed a surface area of about 100 mm2. This early Molteno implant was sutured to the sclera in the anterior segment of the eye at the limbus and the drainage tube was inserted into the anterior chamber of the eye. Once implanted, the body forms scar tissue around this plate. Fluid causes the tissues above the plate to lift and form a bleb into which aqueous humor flows from the anterior chamber via the drainage tube. A bleb is a fluid filled space surrounded by scar tissue, somewhat akin to a blister. The fluid within the bleb then flows through the scar tissue, at a rate which desirably regulates IOP.
Many problems occurred with the early Molteno device. The bleb that formed on the sclera was elevated, which resulted in Dellen formation (sterile corneal ulcers). The implant sometimes had to be removed in another surgery to cure the ulcers. Further, this early device often did not reduce the IOP enough to treat the glaucoma without the use of additional medications.
Dr. Molteno redesigned his implant for insertion into the posterior segment of the eye to avoid the problems with his early anterior device, as disclosed in U.S. Pat. No. 4,457,757 entitled “Device for Draining Aqueous Humor.” This implant is commercially available as the Molteno® Seton Implant and is also referred to as the long tube Molteno implant. The implant comprises a flexible drainage tube connected to one or more rigid plate reservoirs. The plates are shaped to conform to the curvature of the eye. The long tube Molteno implant is disadvantageous as the plates are formed of a rigid plastic which makes insertion beneath the eye tissue difficult and time-consuming. The reservoir plate is placed under Tenon's capsule in the posterior segment of the eye and sutured to the sclera. The drainage tube is implanted into the anterior chamber through a scleral incision.
After a glaucoma implant is attached, IOP tends to fall as aqueous fluid flows immediately through the drainage tube. An open drainage tube may release too much of the fluid too fast, which is detrimental to the eye. It is not until 2-6 weeks later that the bleb forms around the plate to sufficiently regulate the fluid flow. Some prior devices have therefore incorporated valves in the fluid drain path designed to function for a limited time until the bleb forms. However, such valved devices sometimes clog later, requiring another surgery.
More recently, U.S. Pat. Nos. 5,476,445 and 6,050,970 to Dr. George Baerveldt, et al. disclose glaucoma implants or shunts featuring a flexible plate that attaches to the sclera and a drainage tube positioned for insertion into the anterior chamber of the eye. A bleb forms around the plate and fluid drains into and out of the bleb to regulate IOP. This type of shunt is sold under the tradename Baerveldt® BG Series of glaucoma implants by Abbott Medical Optics (AMO) of Santa Ana, Calif. The Baerveldt® device has an open tube with no flow restricting elements. Temporary sutures are used to restrict fluid flow for a predetermined period after which the bleb forms and fluid drainage is properly regulated. The temporary sutures are either biodegradable or removed in a separate procedure. This method works well, but the timing of suture dissolution is necessarily inexact, and a second procedure undesirable.
The Baerveldt® shunts also include four fenestrations (perforations or holes) through the plate to promote fibrous adhesion, which may reduce bleb height. Though a bleb is thought to have a beneficial function in regulating aqueous humor diffusion, too large of a bleb may cause the patient some pain or may be aesthetically unacceptable. Some doctors even prefer to use anti-proliferatives such as mitomycin C or 5-FU at the time of surgery to prevent formation of the fibrous bleb. Another potential complication is endophthalmitis, or an inflammation of the internal tissue of the eye. This complication may occur in any intraocular surgery, with possible loss of vision and the eye itself. Infectious etiology is the most common cause, and various bacteria and fungi have been isolated as the cause of the endophthalmitis. The risk of infection is more pronounced early in a glaucoma shunt procedure when a passage to the interior of the eye is created and fluid flows therethrough. Later, a bleb forms which acts as a filter to prevent microorganisms such as bacteria from entering the eye.
Despite accepted treatments for regulation of intraocular pressure using glaucoma shunts, there is a need for an implant that better manages 1) the configuration of the bleb and attendant aqueous flow performance, and 2) flow performance directly after implantation and before bleb formation.